Optical communication systems often employ Wavelength Division Multiplexing (WDM) with channel plans involving fixed sized allocations of spectrum per channel, each at an assigned center wavelength. Each channel is thus reserved or allocated a specific portion of the optical spectrum such that multiple channels may be transmitted simultaneously through a fiber optical strand or other medium, including free space, thus providing an information carrying capacity.
Channel schemes which employ fixed spectral width allocations may lead to inefficient use of the available spectrum. High data rate channels may, for example, require a larger amount of spectrum while lower rate channels may be able to employ smaller amounts of spectrum. In this mixed rate environment, employing adaptable or variable spectral width allocations instead of fixed spectral width allocations may improve spectrum efficiency. One technique for this is proposed by Jinno, et al in “Spectrum-Efficient and Scalable Elastic Optical Path Network: Architecture. Benefits. and Enabling Technologies” included herein by reference. This improves spectral efficiency by avoiding unused portions within fixed spectral width allocations.
However, this variable spectral width allocation scheme, or any allocation scheme that uses variable sized allocations in a contiguous spectral range per channel can lead to the case where unallocated spectrum may become interspersed with allocated spectrum leading to “stranded” spectrum. That is, aggregation(s) of unallocated spectrum may be sufficient to handle additional allocation demands, but fragmentation may mean that there is no adequate contiguous portion. This reduces the spectrum utilization. When initially planning and deploying a channel plan for a given optical path, the operator will attempt to allocate spectrum for channels such that there is no stranded spectrum between channels: keeping unallocated spectrum contiguous. However, as a client moves channels to alternate optical paths, or changes capacity requirements on any given channel in the optical path, stranded spectrum becomes a real problem. As shown in FIG. 1, spectrum allocations may vary and unallocated spectrum may not be contiguous. For example, the spectrum allocation amounts shown by 123, 101, and 112 each differ, and the unallocated portions of spectrum indicated by 131, 132, 133 and 134 arc not contiguous. Although the cumulative amount of unallocated spectrum 131 through 134 inclusive is equivalent to the allocation assigned in 123, they are not contiguous, and it would not be possible to allocate those portions as shown to a single channel with spectrum needs similar to those allocated to region 123.
If multiple portions of unallocated spectrum are re-arranged so that they collectively form a larger contiguous spectrum, such contiguous spectrum may be allocated to meet a demand, in which case those portions are said to be “recovered”.
The need to recover stranded spectrum may also exist when other techniques are used to improve spectrum utilization. For example, Orthogonal Frequency Division Multiplexing (OFDM) may improve spectrum utilization by increasing the amount of information which may be transported using a given quantity of spectrum, and perhaps allow the spectral widths of channels to vary, but that technique itself does nothing to recover stranded spectrum. Likewise, the use of techniques such as modulation formats and polarization division multiplexing may improve the efficiency of spectrum utilization, but they do not inherently recover stranded spectrum.
Since the amount of usable spectrum is typically limited, it is important to use that spectrum efficiently. Therefore, it is desirable to provide methods and systems for collecting unallocated spectrum into larger contiguous ranges thereby recovering this wasted but valuable resource.